1. The inner envelope, with the names of the invited guests (sometimes omitted in informal invitations)
2. The outer envelope return address, with the name and address of the place you want gifts to be sent to
3. The outer envelope guest address, with the name and address of the guest(s) you’re sending the invitation to

There are several ways to address your invitations, in order of formality:

1. Caligraphy by yourself or someone you hire
2. Handwritten by yourself (or a team of you and your friends and family at an addressing party, my personal favorite)
3. Printed directly on the envelopes by the printer who prints the invitations (only available for return addresses)
4. Printed directly on the envelopes at home using your computer
5. Printed address labels (we recommend clear labels so you don’t have to find ones that match your envelopes)

You can use more than one method of addressing in combination.  For instance, you can have your printer print the return addresses in matching ink and font to your invitations, and then handwrite the addresses and inner envelope names or use labels.

I recommend making the decision about the method of addressing your invitations based on a combination of the formality of your wedding (remember, your invitations set the tone for your event), the formality of your guests (your grandmother may be shocked at seeing her address printed on a label), your budget, and the amount of time you have to dedicate to the task.  If you have the budget and you are having a very formal wedding, hire someone to do the addresses in caligraphy.  If you have a small budget, but have lots of time and a formal wedding, write the addresses yourself.  If you’re in college, have little money, are having a medium-formality wedding, and are planning to get married right after finals, by all means print those labels and slap them on.  It’s better to get the invitations to the guests less formally than to not invite them at all because you’re trying to handwrite the addresses in between classes and just didn’t have the time to finish the task.

UPDATE: My new favorite method is to find someone who can do short-run variable-data envelope printing.  There are few who can do this, and in Nothern California I may be the only one, but it basically means people come to me all day saying “I want the names and addresses printed on the front in the same font and color as the writing on the inside.”  When you call around, people will say “of course we can do envelope printing,” and what they mean is they’re picturing putting it through a press (which won’t work if you’re only printing one of each envelope) or they think they can put it through a copy machine (which will melt the “gum” and seal your envelopes shut, just like your home printer will.)  The machine I have is made by Xanté, and you could probably call them and ask where there is a similar machine in your state.  Or, as always, call me and I’ll get it done and done right.